As I continue to use my Spot, I continue to learn more about the device's pros and cons. A lot of people are reporting difficulty sending messages, for example. In my experience, Spot does not do well transmitting while in motion. It seems to need to stay in one place (motionless is best), with a clear view of the sky, for up to 20 or 30 minutes to send out a message. That makes the tracking function nearly (but not completely) useless, as the tracking pings are supposed to go out every ten minutes automatically.
If you follow these somewhat restrictive rules, I don't think you'll have much trouble sending out 'OK' messages—I haven't had any problems. But maybe the larger question is, what happens when you do send a rescue message? Given that Spot only lets you say either, 'I'm okay' or an ambiguous 'I need Help', it's no small matter to push the help button. Your loved ones won't know that you've twisted your ankle and can't walk back to your car, for example. They'll be thinking you've fractured a femur and are bleeding out (or some other horrific scenario). For a very vivid illustration of this reality (and its consequences), check out Backpacker Magazine's new Blog 'The Pulse', featuring an entry describing from the point of view of those at home what it's like to get a 'Need Help" email: Steve Howe's Rescue Call from Denali.